SpaceX achieved an important milestone over the weekend with successful pressure tests of its Starship SN4 prototype at Boca Chica, clearing the way for an engine static-fire and, perhaps soon, a low-altitude test flight.

Late on April 26, the company’s CEO, Elon Musk, posted a brief Twitter video from a vantage point beneath the SN4 as the towering cylinder shed ice crystals following a successful cryogenic pressurization test meant to simulate the extremely cold fuel conditions that Starship would endure in space.

“Great work by SpaceX engineering!” he tweeted again. The critical cryogenic test, which encased much of the stainless-steel-clad ship in frost, was preceded by an equally successful ambient pressurization test over the weekend. Musk tweeted early Monday that a static-test of the SN4’s single Raptor engine can take place “hopefully later this week.”

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Development of Starship, which the company is designing to carry humans to the Moon and Mars as well as Earth destinations, hasn’t been entirely smooth, though rocket science never is. Starship Mk1, the gleaming, finned prototype that lured countless sightseers to SpaceX’s roadside rocket works near Boca Chica Beach east of Brownsville, was destroyed during a “cryo proof” in November. The subsequent prototype, SN1, failed the same test on Feb. 28, delaying plans for a suborbital test flight this spring.

SN2 passed its pressurization testing on March 8, but was retired to make way for SN3, which collapsed — due to a “test configuration mistake,” according to Musk — on April 2. Then came SN4 and success.

Cameron County had ordered the closure of Boca Chica Beach and S.H. 4 from FM 1419 to the beach on April 25 from 9 p.m. to 6 a.m. the following day for “test launch activities,” with alternate closure dates of April 26 and/or April 27 from 9 p.m. to 6 a.m. the following day. Closure dates were also announced for April 29 from 9 p.m. to 6 a.m. the following day, with alternative dates of April 30 and/or May 1 from 9 p.m. to the following day.

The Federal Aviation Administration has issued Temporary Flight Restrictions up to 1,500 feet around the Boca Chica site covering the same dates.

SpaceX broke ground at the Boca Chica site in September 2014, though only in recent years did site development begin in earnest. The company reached another major milestone on Aug. 27 last year, when the stubby, early Starship prototype “Starhopper” flew to an altitude of 500 feet before setting down in a controlled landing, in a flight lasting 57.36 seconds.